Semiconductor device

ABSTRACT

A semiconductor device has an active layer, a first semiconductor layer of first conductive type, an overflow prevention layer disposed between the active layer and the first semiconductor layer, which is doped with impurities of first conductive type and which prevents overflow of electrons or holes, a second semiconductor layer of first conductive type disposed at least one of between the active layer and the overflow prevention layer and between the overflow prevention layer and the first semiconductor layer, and an impurity diffusion prevention layer disposed between the first semiconductor layer and the active layer, which has a band gap smaller than those of the overflow prevention layer, the first semiconductor layer and the second semiconductor layer and which prevents diffusion of impurities of first conductive type.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2005-247838, filed on Aug. 29,2005, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device containinggallium nitride (GaN)-based compound semiconductor.

2. Related Art

Gallium nitride-based (GaN-based) semiconductors have wide band gaps,and the characteristics of GaN-based semiconductors have been used inresearch and development of high-brightness ultraviolet to blue/greenLEDs and violet laser diodes. Further, high-frequency and high-power GaNtransistors or the like have been fabricated.

In a GaN-based semiconductor, since an effective mass of an electron ora positive hole is larger than that of a GaAs-based semiconductor, atransparent carrier density of the GaN-based laser is larger than thatof GaAs-based laser. Therefore, a threshold current density of aGaN-based laser is inevitably higher than that of a GaAs-based laser. Arepresentative value of the threshold current density of the GaN-basedlaser is about 1 to 3 kAcm⁻².

As described above, since a GaN-based laser has a high threshold currentdensity, it is critically important to suppress overflow of carriers(particularly electrons). In a GaN-based laser, a GaAIN layer doped withp-type impurity is often disposed near an active layer to suppressoverflow of electrons (Shuji Nakamura et al., “InGaN-BasedMulti-Quantum-Well-Structure Laser Diodes”, Japanese Journal of AppliedPhysics, Jan. 15, 1996, volume 35, No. 1B, pp. L74-L76, M. Hansen etal., “Higher efficiency InGaN laser diodes with an improved quantum wellcapping configuration”, Applied Physics Letters, Nov. 25, 2002, volume81, No. 22, pp. 4275-4277).

However, during crystal growth of an actual device structure, InGaN andGaN/GaAlN used as guide layer materials are grown at differenttemperatures. The growth temperature of InGaN is about 700 to 800° C.,whereas the growth temperature of GaN/GaAlN is 1000 to 1100° C. In otherwords, after InGaN is grown, the growth is suspended, InGaN undergoes atemperature rising process, and then GaN/GaAlN is grown. It has beenfound that a defect caused by heat damage is introduced to a crystalgrowth layer in this temperature rising process. When the layer withsuch a defect is arranged close to an active layer, the life of thedevice may decrease. Therefore, in order to achieve a highly reliabledevice, it is important to locate the layer with such a defect away fromthe active layer.

When a GaAIN layer doped with p-type impurity is arranged quite close toan active layer, the p-type impurity causes a free carrier loss and, onthe contrary, increases a threshold current density. Further, the p-typeimpurity may diffuse to the active layer. In this case, the lossincreases and the threshold current density also increases. Even if thediffusion of p-type impurity to the active layer is suppressed in theinitial stage of energization of the laser diode, the impurity maydiffuse to the active layer during a life test with a constant opticaloutput, so that the threshold current density may increase and the laserdiode may be finally disabled. In this way, the diffusion of p-typeimpurity to the active layer is a serious problem to the reliability ofthe device.

SUMMARY OF THE INVENTION

The present invention provides a semiconductor device which can preventimpurity from diffusing to an active layer.

According to one embodiment of the present invention, a semiconductordevice, comprising:

an active layer;

a first semiconductor layer of first conductive type;

an overflow prevention layer disposed between the active layer and thefirst semiconductor layer, which is doped with impurities of firstconductive type and which prevents overflow of electrons or holes;

a second semiconductor layer of first conductive type disposed at leastone of between the active layer and the overflow prevention layer andbetween the overflow prevention layer and the first semiconductor layer;and

an impurity diffusion prevention layer disposed between the firstsemiconductor layer and the active layer, which has a band gap smallerthan those of the overflow prevention layer, the first semiconductorlayer and the second semiconductor layer and which prevents diffusion ofimpurities of first conductive type,

wherein each of the active layer, the overflow prevention layer, thefirst semiconductor layer, the second semiconductor layer and theimpurity diffusion prevention layer are formed of GaN-based compoundsemiconductor.

According to one embodiment of the present invention, a semiconductordevice, comprising:

an active layer;

a first semiconductor layer of first conductive type;

an overflow prevention layer disposed between the active layer and thefirst semiconductor layer, which is doped with impurities of firstconductive type and which prevents overflow of electrons or holes;

a second semiconductor layer of first conductive type which is disposedeither of between the active layer and the overflow prevention layer orbetween the overflow prevention layer and the first semiconductor layer;and

an impurity diffusion prevention layer disposed between the overflowprevention layer and the second semiconductor layer, which has a bandgap smaller than those of the overflow prevention layer, the firstsemiconductor layer and the second semiconductor layer and whichprevents diffusion of impurities of first conductive type,

wherein each of the active layer, the overflow prevention layer, thefirst semiconductor layer, the second semiconductor layer and theimpurity diffusion prevention layer are formed of GaN-based compoundsemiconductor.

According to one embodiment of the present invention, a semiconductordevice, comprising:

an active layer;

a first semiconductor layer of first conductive type;

an overflow prevention layer disposed between the active layer and thefirst semiconductor layer, which is doped with impurities of firstconductive type and which prevents overflow of electrons or holes;

a second semiconductor layer of first conductive type disposed betweenthe overflow prevention layer and the first semiconductor layer;

a third semiconductor layer of first conductive type disposed betweenthe active layer and the overflow prevention layer; and

an impurity diffusion prevention layer disposed at least one of betweenthe overflow prevention layer and the second semiconductor layer andbetween the overflow prevention layer and the third semiconductor layer,which has a band gap smaller than those of the overflow preventionlayer, the first semiconductor layer and the second semiconductor layerand which prevents diffusion of impurities of first conductive type,

wherein each of the active layer, the overflow prevention layer, thefirst semiconductor layer, the second semiconductor layer and theimpurity diffusion prevention layer are formed of GaN-based compoundsemiconductor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing a semiconductor device according toEmbodiment 1 of the present invention;

FIG. 2 is a diagram showing the relationship between a depth of alaminated film and a Mg concentration and the relationship between adepth and band gap energy;

FIGS. 3A-3D are process drawings showing the manufacturing process of alaser diode shown in FIG. 1;

FIGS. 4A-4C are process drawings following FIG. 3;

FIG. 5 is a sectional view showing a semiconductor device in which animpurity diffusion prevention layer 8 is interposed between an overflowprevention layer 7 and a p-type first guide layer 6;

FIG. 6 is a sectional view showing a laser diode of Embodiment 2;

FIG. 7 is a sectional view showing a laser diode in which an impuritydiffusion prevention layer 8 is interposed between a p-type GaN guidelayer 21 and an overflow prevention layer 7;

FIG. 8 is a sectional view showing a laser diode in which the impuritydiffusion prevention layer 8 is interposed between a p-type clad layer10 and the p-type GaN guide layer 21; and

FIG. 9 is a sectional view showing a laser diode in which the impuritydiffusion prevention layer 8 is interposed between the p-type GaN guidelayer 21 and the overflow prevention layer 7.

DETAILED DESCRIPTION OF THE INVENTION

Hereafter, a receiver and a receiving method according to the presentinvention will be described more specifically with reference to thedrawings.

Exemplary embodiments of the present invention will now be describedwith reference to the accompanying drawings.

Embodiment 1

FIG. 1 is a sectional view of a semiconductor device according toEmbodiment 1 of the present invention. FIG. 1 shows a semiconductorlight-emitting device, to be specific, the cross-sectional structure ofa laser diode. The laser diode shown in FIG. 1 includes an n-type GaNbuffer layer 2 formed on an n-type GaN substrate 1, an n-type clad layer3 formed thereon, an n-type guide layer 4 formed thereon, an activelayer 5 formed thereon, a p-type first guide layer 6 formed thereon, aGa_(x)Al_(1-x)N (0<x≦1) layer (overflow prevention layer) 7 formedthereon, an In_(y)Ga_(1-y)N (0<y≦1) layer (impurity diffusion preventionlayer) 8 formed thereon, a p-type GaN second guide layer 9 formedthereon, a p-type clad layer 10 formed thereon, and a p-type contactlayer 11 formed thereon. The overflow prevention layer can be extendedmore generally to In_(1-x-y)Ga_(x)Al_(y)N (0≦x<1, 0<y≦1).

The composition ratio of In in the impurity diffusion prevention layer 8is set higher than those of the overflow prevention layer 7, the guidelayers 6 and 9 and the p-type clad layer 10. As a guide of diffusion ofimpurity (described later), the composition ratio of In in the impuritydiffusion prevention layer 8 is set at 2% to 10%, preferably 3% to 8%,and the composition ratio of In in the overflow prevention layer 7 andthe guide layers 6 and 9 is set at 2% or less. Generally, as thecomposition ratio of In increases, the index of refraction increases andthe band gap decreases. When the composition ratio of In is low in theimpurity diffusion prevention layer 8, it is difficult to preventdiffusion of impurity. In view of luminous efficiency, it is desirablethat the composition ratio of In in the impurity diffusion preventionlayer 8 is lower than that of the quantum well layer of the activelayer.

The p-type clad layer 10 has a convex portion. The p-type GaN contactlayer 11 is formed on the top surface of the convex portion, and aninsulating layer 12 is formed on the side walls of the convex portionand a surface of the p-type clad layer 10 except for the convex portion.A p-type electrode 13 is formed on the p-type contact layer 11 and ann-type electrode 14 is formed on the backside of the n-type GaNsubstrate 1.

The laser diode of FIG. 1 has the impurity diffusion prevention layer 8between the overflow prevention layer 7 and the p-type GaN second guidelayer 9. The impurity diffusion prevention layer 8 absorbs p-typeimpurity which is present in the p-type GaN guide layer 9 and the p-typeclad layer 10 and so on, so that the p-type impurity does not diffuse tothe active layer. Although diffusion of impurity can be sufficientlyprevented by disposing the impurity diffusion prevention layer 8 closeto the p-type clad layer 10, diffusion of impurity can be furtherprevented by disposing the impurity diffusion prevention layer 8 closeto the active layer 5. The reason is that it is possible to preventdiffusion of p-type impurity included in as many as possible of one ormore p-type semiconductor layers between the active layer 5 and thep-type clad layer 10. However, when the impurity diffusion preventionlayer 8 is in contact with the active layer 5, a quantum well layer ofthe active layer 5 has a smaller band gap than the impurity diffusionprevention layer 8. Thus the p-type impurity may not be sufficientlyabsorbed by the impurity diffusion prevention layer 8 but diffused tothe active layer 5, which is not desirable.

The inventor examined the doping profile of p-type impurity (forexample, Mg) in a laminated film made of GaN, GaAlN and InGaN by usingsecondary ion-microprobe mass spectrometry (SIMS). As a result, it wasfound that in spite of a constant doping concentration, InGaN has thehighest Mg concentration even in consideration of the matrix effect ofSIMS. FIG. 2 shows this result. In chart “a” of FIG. 2, the horizontalaxis represents a depth position of the laminated film and the verticalaxis represents a concentration of Mg. In chart “b”, the horizontal axisrepresents a depth position of the laminated film and the vertical axisrepresents band gap energy.

As indicated by chart “a” of FIG. 2, the impurity diffusion preventionlayer 8 has the highest Mg concentration. Further, chart “b” indicatesthat the impurity diffusion prevention layer 8 has the lowest band gapenergy.

The reason why InGaN has a high Mg concentration is that InGaN is largerin lattice constant than GaN and GalN (strictly saying, a latticeconstant is large in the c-axis direction) and Mg is easily extractedinto the film.

As shown in FIG. 2, a Mg concentration and band gap energy arecorrelated with each other in each layer of the laminated film.Therefore also in the laser diode of FIG. 1, the In_(y)Ga_(1-y)Nimpurity diffusion prevention layer 8 is made of a material having lowerband gap energy than the overflow prevention layer 7 and the p-type GaNsecond guide layer 9 which are disposed on the respective sides of theimpurity diffusion prevention layer 8, so that p-type impurity containedin the p-type GaN second guide layer 9, the p-type clad layer 10, and soon can be accumulated in the impurity diffusion prevention layer 8. Inother words, the lattice constant of the impurity diffusion preventionlayer 8 in the c-axis direction is made larger than those of theoverflow prevention layer 7 and the p-type GaN second guide layer 9 inthe c-axis direction, the layers 7 and 9 being disposed on therespective sides of the impurity diffusion prevention layer 8. Thusp-type impurity can be accumulated in the impurity diffusion preventionlayer 8.

In the present embodiment, the impurity diffusion prevention layer 8including an In_(y)Ga_(1-y)N layer having a smaller band gap is disposedon the overflow prevention layer 7 doped with p-type impurity and thep-type impurity is accumulated in the impurity diffusion preventionlayer 8, so that the p-type impurity is not diffused to the activelayer.

FIGS. 3 and 4 are process drawings showing the manufacturing process ofthe laser diode shown in FIG. 1. First, on the n-type GaN substrate 1,the crystal of the n-type GaN buffer layer 2 doped with n-type impurityis grown (FIG. 3(A)). For example, Metal Organic Chemical VaporDeposition (MOCVD) is used for the crystal growth. Further, MolecularBeam Epitaxy (MBE) may be used for the crystal growth. The n-typeimpurity may be Si or Ge. Si is used in the present embodiment.

And then the superlattice n-type clad layer 3 including an undopedGa_(0.9)Al_(0.1)N layer and a GaN layer doped with an n-type impurity ofabout 1×10¹⁸ cm⁻³ is grown on the n-type GaN buffer layer 2 (FIG. 3(B)).The material of the n-type clad layer 3 is not particularly limited. Forexample, a thick film of Ga_(0.95)Al_(0.05)N may be used. Alternativelyboth of the Ga_(0.9)Al_(0.1)N layer and the GaN layer may be doped withn-type impurity to form the n-type clad layer 3.

And then the n-type guide layer 4 made of GaN with a thickness of about0.1 μm is grown on the n-type clad layer 3. The n-type guide layer 4 isdoped with n-type impurity of about 1×10¹⁸ cm⁻³. Alternatively then-type guide layer 4 may be made of In_(0.01)Ga_(0.99)N with a thicknessof about 0.1 μm. The n-type GaN buffer layer 2, the n-type clad layer 3,and the n-type guide layer 4 are grown at 1000 to 1100° C.

And then on the n-type guide layer 4, the active layer 5 having amultiple quantum well (MOW) structure is formed (FIG. 3(C)). In thisstructure, quantum well layers each of which includes an undopedIn_(0.01)Ga_(0.9)N layer having a thickness of about 3.5 nm and barrierlayers each of which includes an undoped In_(0.01)Ga_(0.99)N layerhaving a thickness of about 7 nm are alternately stacked such that thebarrier layers are disposed on both sides of the quantum well. In thiscase, the growth temperature is 700 to 800° C.

And then the p-type first guide layer 6 made of In_(0.005)Ga_(0.995)N isgrown on the active layer 5. The p-type first guide layer 6 onlyrequires a thickness of about 90 nm. The p-type first guide layer 6 maybe undoped or doped with about 1×10¹⁷ cm⁻³ to about 5×10¹⁸ cm⁻³ of Mg.Mg is a p-type impurity. When the n-type guide layer 5 disposed underthe active layer is made of GaN or In_(x1)Ga_(1-x1)N (0<x1<1) and theactive layer has a single or multiple quantum well structure including aquantum well containing In_(x2)Ga_(1-x2)N (0<x2≦1) and a barrier layercontaining In_(x3)Ga_(1-x3)N (0≦x3<1, x2>x3), the p-type first guidelayer 6 is made of In_(x4)Ga_(1-x4)N (0≦x4<1, x3>x4).

And then a Ga_(0.8)Al_(0.2)N layer having a thickness of about 10 nm isgrown on the p-type first guide layer 6. The Ga_(0.8)Al_(0.2)N layer isdoped with about 4×10¹⁸ cm⁻³ to about 5×10¹⁹ cm⁻³ of Mg. TheGa_(0.8)Al_(0.2)N layer is provided to prevent overflow of electrons andthus also called the overflow prevention layer 7. The p-type first guidelayer 6 and the overflow prevention layer 7 are grown at 1000 to 1100°C.

And then the impurity diffusion prevention layer 8 made ofIn_(y)Ga_(1-y)N (0<y≦1) is grown on the overflow prevention layer 7(FIG. 3(D)). Composition y of In represents, for example, y=0.02 with athickness of 3 nm. The thickness is set at, for example, 1 to 15 nm andpreferably set at 1 nm to 10 nm. A small thickness makes it difficult toobtain the effect of preventing impurity diffusion, and a largethickness changes the light intensity distribution, which is notdesirable. The impurity diffusion prevention layer 8 is grown preferablyat 700° C. to 800° C. In the case of a low In content (for example, 3%or less), the impurity diffusion prevention layer 8 may be grown at 1000to 1100° C. The impurity diffusion prevention layer 8 may be doped withabout 1×10¹⁷ cm⁻³ to about 1×10^(19 cm) ⁻³ of Mg.

And then the p-type GaN second guide layer 9 doped with about 2×10¹⁸cm⁻³ to about 5×10¹⁹ cm⁻³ of Mg is grown on the In_(y)Ga_(1-y)N layer.This layer is, for example, 0.05 μm in thickness. Subsequently thep-type clad layer 10 having a superlattice structure is grown on thep-type GaN second guide layer 9. The superlattice structure includes anundoped Ga_(0.9)Al_(0.1)N layer and GaN doped with about 1×10¹⁹ cm⁻³ toabout 5×10¹⁹ cm⁻³ of Mg. The material of the p-type clad layer 10 is notparticularly limited. The p-type clad layer 10 may be a thick film(about 0.6 μm in thickness) doped with p-type impurity including, forexample, Ga_(0.95)Al_(0.05)N. Alternatively both of Ga_(0.9)Al_(0.1)Nand GaN may be doped with p-type impurity. And then the p-type contactlayer 11 including a GaN layer doped with p-type impurity with athickness of 0.1 μm is formed on the p-type clad layer 10 (FIG. 4(A)).Instead of the GaN layer, an InGaAlN layer doped with p-type impuritymay be used. The p-type GaN second guide layer 9, the p-type clad layer10, and the p-type contact layer 11 are grown at 1000° C. to 1100° C.

A device process is performed on a wafer where crystal has been grownaccording to the process of FIGS. 3 to 4(A), so that a laser diode isfinally formed. The p-type contact layer 11 and the p-type clad layer 10are partially removed by lithography and dry etching to form a ridgestructure having a convex portion (FIG. 4(B)). Further, the insulatinglayer 12 is formed on the side walls of the convex portion and a surfaceof the p-type clad layer 10 except for the convex portion (FIG. 4(C)).

And then the p-type electrode 13 is formed on the insulating layer 12and the p-type GaN contact layer 11 doped with about 3×10¹⁹ cm⁻³ toabout 1×10²² cm⁻³ of Mg, and the n-type electrode 14 is formed on thebackside of the n-GaN substrate.

The end face of the laser diode is formed by cleavage and a coating witha high reflectivity is applied on a surface opposite from the lightextracting surface.

A convex laminated structure including the p-type clad layer 10 and thep-type GaN contact layer 11 extends in the vertical direction of thedrawing and acts as a resonator.

The shape of the convex laminated structure is not limited to arectangle having a vertical side wall in the cross section of FIG. 1.The structure may have a trapezoidal convex portion with a slope of amesa. The p-type contact layer 11 is about 2 μm in width (ridge width)and the resonator length is set at, for example, 600 μm.

On the side walls of the convex portion and the surface of the p-typeclad layer 10 except for the convex portion, a current block layerincluding the insulating layer 12 is formed with the convex portioninserted in the insulating layer 12. The current block layer controlsthe transverse mode of the laser diode. Although the thickness of thecurrent block layer can be arbitrarily selected according to a design,the thickness is preferably set at about 0.3 μm to 0.8 μm, for example,about 0.5 μm.

The material of the current block layer includes, for example, ahigh-resistivity semiconductor film such as an AlN film and aGa_(0.8)Al_(0.2)N film, a semiconductor film irradiated with proton, asilicon oxide film (SiO₂ film), and a multilayer film made up of a SiO₂film and a ZrO₂ film. In other words, various materials can be used forthe current block layer as long as the materials are lower in the indexof refraction than a nitride III-V compound semiconductor used for theactive layer 5.

Moreover, the laser diode of the present embodiment does not always haveto have a ridge waveguide laser structure. For example, in the case ofan embedded laser structure, an n-type semiconductor layer such asn-type GaN and n-type GaAlN may be used, instead of an insulating film,as the current block layer by pn junction isolation.

The p-type electrode 13 including, for example, a composite film ofpalladium-platinum-gold (Pd/Pt/Au) is formed on the p-type GaN contactlayer 11. For example, a Pd film is 0.05 μm in thickness, a Pt film is0.05 μm in thickness, and an Au film is 1.0 μm in thickness.

On the other hand, the n-type electrode 14 including, for example, acomposite film of titanium-platinum-gold (Ti/Pt/Au) is formed on thebackside of the n-type GaN substrate 1. For the n-type electrode 14, forexample, a Ti film having a thickness of 0.05 μm, a Pt film having athickness of 0.05 μm, and an Au film having a thickness of 1.0 μm areused.

The laser diode manufactured by the manufacturing process of FIGS. 3 and4 has a threshold current of 35 mA on the average in a current-opticaloutput characteristic. Also in a laser diode where the impuritydiffusion prevention layer 8 is not provided on the overflow preventionlayer 7, the threshold current is about 35 mA on the average. Thereforeit is understood that the presence or absence of the impurity diffusionprevention layer 8 does not cause a different initial characteristic ofthe laser diode.

The inventor conducted an energization test in which a life is measuredwith a constant optical output. In this conduction test, the laser diodewas caused to continuously oscillate with an optical output of 50 mW andan operating temperature of 75° C. to examine the rate of increase ofthe operating current. A time period during which the operating currentincreases from the initial value by 20% is defined as the life of thelaser diode. The life of the laser diode of FIG. 1 was measuredaccording to this definition, so that the life was estimated to be 1000hours or longer according to a change in the rate of increase. On theother hand, the life of the laser diode not having the impuritydiffusion prevention layer 8 was estimated to be 200 to 300 hours.

The cause of the difference in life will be discussed below. When theimpurity diffusion prevention layer 8 is omitted, during theenergization test, p-type impurity (for example, Mg) in the p-type cladlayer 10 and the p-type second guide layer 9 gradually starts diffusingto the active layer 5 containing less impurity. The p-type impuritydiffused to the active layer 5 causes a free carrier loss, and thus thethreshold current increases in the laser diode. Further, slopeefficiency which indicates a ratio of a change in optical output to achange in current at the threshold current or higher decreases.Therefore the operating current increases when the optical output iskept constant.

When the impurity diffusion prevention layer 8 is provided as in thepresent embodiment, p-type impurity is accumulated in the impuritydiffusion prevention layer 8, thereby suppressing the diffusion of thep-type impurity to the active layer 5. It is thus possible to provide alaser diode with a long life and high reliability.

While the impurity diffusion prevention layer 8 is interposed betweenthe overflow prevention layer 7 and the p-type GaN second guide layer 9in the laser diode of FIG. 1, the impurity diffusion prevention layer 8may be interposed between the overflow prevention layer 7 and the p-typefirst guide layer 6 as shown in FIG. 5. Also in FIG. 5, p-type impurityin the overflow prevention layer 7 can be positively accumulated in theimpurity diffusion prevention layer 8.

As described above, the impurity diffusion prevention layer 8 made ofIn_(y)Ga_(1-y)N is disposed near the active layer 5 in the presentembodiment, and thus p-type impurity in the p-type clad layer 10 or thep-type second guide layer 9 can be accumulated in the impurity diffusionprevention layer 8 and does not diffuse to the active layer 5. It isthus possible to increase the life of the laser diode and improvereliability.

Embodiment 2

Embodiment 2 is different from Embodiment 1 in the structure of thelaser diode.

FIG. 6 is a sectional view showing the laser diode of Embodiment 2. Thelaser diode of FIG. 6 has a p-type GaN guide layer 21 which is a singlelayer combining the p-type first guide layer 6 and the p-type GaN secondguide layer 9 of FIG. 1. The guide layer is interposed between an activelayer 5 and an overflow prevention layer 7. To be specific, the laserdiode of FIG. 6 includes an n-type GaN buffer layer 2 formed on ann-type GaN substrate 1, an n-type clad layer 3 formed thereon, an n-typeguide layer 4 formed thereon, the active layer 5 formed thereon, thep-type GaN guide layer 21 formed thereon, the Ga_(0.8)Al_(0.2)N layer(overflow prevention layer 7) formed thereon, an In_(y)Ga_(1-y)N (0<y≦1)layer (impurity diffusion prevention layer 8) formed thereon, and ap-type clad layer 10 formed thereon.

In the laser diode of FIG. 6, p-type impurity in the p-type clad layer10 can be accumulated in the impurity diffusion prevention layer 8, andthus it is possible to prevent the p-type impurity from diffusing to theactive layer 5.

In FIG. 6, the impurity diffusion prevention layer 8 is interposedbetween the overflow prevention layer 7 and the p-type clad layer 10.The impurity diffusion prevention layer 8 may be interposed between thep-type GaN guide layer 21 and the overflow prevention layer 7 as shownin FIG. 7.

In the laser diode of FIG. 7, p-type impurity in the overflow preventionlayer 7 as well as the p-type clad layer 10 can be accumulated in theimpurity diffusion prevention layer 8.

In the laser diodes of FIGS. 6 and 7, the order of stacking the p-typeGaN guide layer 21 and the overflow prevention layer 7 may be reversed.In this case, a laser diode shown in FIG. 8 or 9 is obtained. In thelaser diode of FIG. 8, the impurity diffusion prevention layer 8 isinterposed between the p-type clad layer 10 and the p-type GaN guidelayer 21. In the diode of FIG. 9, the impurity diffusion preventionlayer 8 is interposed between the p-type GaN guide layer 21 and theoverflow prevention layer 7.

As described above, in any of these structures shown in FIGS. 6 to 9,p-type impurity can be accumulated in the impurity diffusion preventionlayer 8. It is thus possible to prevent p-type impurity from diffusingto the active layer and increase the life of the laser diode.

In Embodiments 1 and 2, the p-type impurity is Mg. Zn or the like may beused.

In Embodiments 1 and 2, there have been described examples in which thelaser diode includes the impurity diffusion prevention layer 8. Thepresent invention is applicable not only to a laser diode but also anoptical device such as a light-emitting diode and a photodetector and anelectronic device such as a transistor (for example, a heterojunctionbipolar transistor (HBT)).

Further, in the embodiments, there have been described examples in whichp-type impurity is accumulated in the impurity diffusion preventionlayer 8. When an n-type guide layer and an overflow prevention layer forpreventing overflow of a positive hole doped with n-type impurity areprovided, the n-type impurity may be accumulated in an impuritydiffusion prevention layer formed adjacent to these layers.

1. A semiconductor device, comprising an active layer, a firstsemiconductor layer of first conductive type, an overflow preventionlayer disposed between the active layer and the first semiconductorlayer, which is doped with impurities of first conductive type and whichprevents overflow of electrons or holes, a second semiconductor layer offirst conductive type which is disposed either between the active layerand the overflow protection layer or between the overflow preventionlayer and the first semiconductor layer, and an impurity diffusionprevention layer disposed between the overflow prevention layer and thesecond semiconductor layer, which has a band gap smaller than those ofthe overflow prevention layer, the first semiconductor layer and thesecond semiconductor layer and which prevents diffusion of impurities offirst conductive type.
 2. The semiconductor of claim 1, wherein thesecond semiconductor layer is disposed between the active layer and theoverflow prevention layer, and wherein the impurity diffusion preventionlayer is disposed between the second semiconductor layer and theoverflow prevention layer.
 3. The semiconductor device of claim 1,wherein the impurity diffusion prevention layer comprises In, and acomposition ratio of In in the impurity diffusion prevention layer ishigher than those of the overflow prevention layer, the firstsemiconductor layer and the second semiconductor layer.
 4. Thesemiconductor device of claim 1, wherein the active layer emits a lightwith a predetermined wavelength, wherein the first conductive type isp-type, wherein the semiconductor layer is used as a p-type clad layer,wherein the second semiconductor layer is used as a p-type guide layer;and wherein the overflow prevention layer prevents overflow ofelectrons.
 5. The semiconductor device of claim 4, further comprising ann-type guide layer disposed on a side of the active layer opposite fromthe p-type guide layer, which comprises GaN or In_(x1)Ga_(1-X1)N(0<x1<1), wherein the active layer comprises a single or multiplequantum well structure having In_(x2)Ga_(1-x2)N (0<x2≦1) and a barrierlayer having In_(x3)Ga_(1-x3)N (0≦x3<1, x2>x3), and wherein the p-guidelayer has In_(x4)Ga_(1-x4)N (0≦x4<1, x3>x4).
 6. The semiconductor deviceof claim 1, wherein the overflow prevention layer has a Ga_(1-y)Al_(y)Nlayer (0<y≦1).
 7. A semiconductor device, comprising an active layer, afist semiconductor layer of first conductive type, an overflowprevention layer disposed between the active layer and the firstsemiconductor layer, which is doped with impurities of first conductivetype and which prevents overflow of electrons or holes; a secondsemiconductor layer of first conductive type disposed between theoverflow prevention layer and the first semiconductor layer, a thirdsemiconductor layer of first conductive type disposed between the activelayer and the overflow prevention layer, and an impurity diffusionprevention layer disposed at least one of between the overflowprevention layer and the second semiconductor layer and between theoverflow prevention layer and the third semiconductor layer, which has aband gap smaller than those of the overflow prevention layer, the firstsemiconductor layer, the second semiconductor layer and the thirdsemiconductor layer and which prevents diffusion of impurities of firstconductive type.
 8. The semiconductor device of claim 7, wherein theimpurity diffusion prevention layer is disposed between the secondsemiconductor layer and the overflow prevention layer.
 9. Thesemiconductor device of claim 7, wherein the impurity diffusionprevention layer is disposed between the third semiconductor layer andthe overflow prevention layer.
 10. The semiconductor device of claim 7,wherein the impurity diffusion prevention layer comprises In, andwherein a composition ration of In in the impurity diffusion preventionlayer is higher than those of the overflow prevention layer, the firstsemiconductor layer, the second semiconductor layer and the thirdsemiconductor layer.
 11. The semiconductor device of claim 7, whereinthe active layer emits a light with a predetermined wavelength, whereinthe first conductive type is p-type, wherein the first semiconductorlayer is used as a p-type clad layer, wherein each of the second andthird semiconductor layers is used as a p-type guide layer, and whereinthe overflow prevention layer prevents overflow of electrons.
 12. Thesemiconductor device of claim 11, further comprising an n-type guidelayer disposed on a side of the active layer opposite from the p-typeguide layer, which comprises GaN or In_(x1)Ga_(1-x1)N (0<x1<1), whereinthe active layer includes a single or multiple quantum well structurehaving In_(x2)Ga_(1-x2)N (0<x2≦1) and a barrier layer havingIn_(x3)Ga_(1-x3)N (0≦x3<1, x2>x3), and wherein the p-guide layer hasIn_(x4)Ga_(1-x4)N (0≦x4<1, x3>x4).
 13. The semiconductor device of claim7, wherein the overflow prevention layer has Ga_(1-y)Al_(y)N layer(0<y≦1).